Now take this scene from the ROOM
The actor Tommy Wiseau is pretty out of form and his body language and Dialogue delivery is pretty mundane which results in a very badly acted and a very unintentionally funny scene.
ON THE FLIP SIDE
Take an actor like Robert De Niro from Taxi Driver.
De Niro played the role with such conviction that you can actually relate to the character of Travis Brickle and you can feel for the internal conflicts faced by the character and his overview and ideologies
It is bad conviction which makes Tommy Wiseau's Performance a pretty bad one and it is the same conviction in a more good form which makes De Niro's role a masterpiece in Cinema History.
Good Conviction leads to two more things which are important which are
BELIEVABILITY
AND
RELATABILITY
So this is why I believe that conviction is pretty important.
3. Versatility: A good actor should be able to take and play as many different and unique roles as possible to maintain variety and to be versatile.
Some versatile actors are as follows :
And many More.
4. Character design: i personally feel that it is the character design which decides whether an actor needs to perform subtly or expressively.
Now take Rajinikanth's Thalapathi for example
The role of Surya was played pretty subtly and as much natural as possible. There was not much dialogues and expressions and yet the character worked out as that was the design thought by ace Filmmaker Mani Ratnam.
Now think how it would have been if the same kind of subtlety was there in a movie like Muthu or Annamalai where the character is expected to be expressive.
It wouldn't have worked that much and it would have flopped big time.
Hence the actors need to understand the ethos of their character and act according to it to give a splendid performance like no other.
CONCLUSION
These are some of my criteria of how to judge an actors performance. I personally believe that both expressive and Natural acting are pretty good and it is the character design like I said earlier which decides whether a character needs to be acted subtley or expressively.
As far as my concern when an actor acts I need to feel his performance to be relatable and I need to see the kind of conviction with which he acts the role to perfection so that I can get invested. An actors role should neither be overpowered and neither should it be undercooked
There should be a perfect and optimum balance to give a solid and memorable performance.